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Image Link and Telescope Pointing
How to refine your telescope's pointing with Image Link.
BackgroundThis page assumes that you are able to take CCD images and get them to Image Link successfully in TheSky. Please refer to the Image Link Explained page for complete details on how to perform an Image Link on your CCD images.
First get the telescope reasonably close to the target. The telescope must be initialized and pointing reasonably well first on its' own. Use Image Link and Sync to begin the imaging session. This is simply the very best most efficient way to get started. Even for TPoint users who are first starting out mapping.
When trying to accurately center certain target positions, say an object imaged the night before, make sure that you Image Link and Sync very close to the target to be slewed to. By syncing say even only 1 degree away from the target your telescope may not point as accurately as you think it should. Why? <----- Please read on!. The closer you sync to the target the better the next object pointing will be. It is that simple. This excludes TPoint users whereby TPoint closes the pointing loop.
So now that the telescope is has been Image Linked and Synced very near the target the next slew will place the target object very close to perfectly centered on your detector (or in your eyepiece). Now let's say you want to go back and image an object taken on a previous night. You can use Image Link to place the telescope back to the same position easily enough. Here is exactly how. Going back to coordinates from a previous imageUsing CCDSoft or similar load an image taken on a previous night. This image will be Edit | Copied to the Clipboard then Edit | Pasted into TheSky and Image Linked. Of course this assumes you have a CCD image to work with. If you don't you will need the coordinates from the night before. These can easily be added to TheSky then slewed to as well. But using Image Link as a tool is much more convenient!
HINT: Before attempting the Image Link on the previous night's image first you should "SUSPEND" the telescope link (do not terminate just suspend). You are going to find a link that is not exactly where the telescope is currently pointing. If you don't do this the screen will jump over to the new linked position then right back to the telescope cross hair position.
You can either disable the option to keep Telescope Cross hair centered or just SUSPEND the link BEFORE linking the image taken previously. Telescope Tool Bar Or just use Telescope | Link | Suspend instead of the telescope tool bar's yellow icon. Or, Disable Telescope Cross
Hair Centering here Now at the point where the image can be Image Linked successfully you have the coordinates of the center of the image accurate to your scale in arcseconds/pixel. So? Well this makes slewing to the position simple! Just use the option to "Slew To" the Image Link coordinates.
NOTE: The Image Link information is NOT the first entry in the object information dialog when you click on TheSky display. In this example here there are 6 objects found when M20 the Trifid nebula is clicked on after an image was linked.
Use the Down Arrow highlighted "V" on the right side of object entry to get to the last entry titled "Image Link Information". Image Link Information end of list Now what? Well, since you have the center of the image provided by Image Link you can simply slew the telescope now to the accurate Image Link coordinates. Assuming you have previously "Synced" the telescope nearby (5 to 10 arcminutes from the target) this will place the object very near center. Again, the closer you Sync to the target the better the pointing will appear to be but only in a small area. For good all sky RMS pointing better to use TPoint modeling software. Highlight Image Link
Information Why can't I just slew to an object by name like slew to M20?
You can. However, let's say the image you took the previous night was refined to find a guide star on the smaller guide chip. Because you had to alter the telescope's position to find the star this is not included in the slew to object position. However, it is factored in when using the Image Link and Sync previous image because the coordinates slewed to are the center of the image regardless of where the telescope was originally slewed to. Meaning the offset in either RA and/or DEC to find the guide star are now factored in for you. Cool. Alternative Centering methodsCan you center objects another way?
Yes. When you have performed an Image Link simply click on any object in the image and then slew to it. Even objects that are not cataloged. Just use the entry "Cursor Position" when there is no object found in TheSky where you have clicked, then slew to "Cursor Position". Click target then slew Or in CCDSoft simply RIGHT mouse click on the target in your CCD image an you will be presented with 3 centering options. Center WCS (World Coordinate System - Astrometric solution) via Telescope Slew, Center Cursor Position using Relays (Calibration required) or Center Brightest object via relays (Calibration required).
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